Stormwater is rainwater plus any particulate debris and dissolved materials that the rainwater carries along with it. In urban areas, rain that falls on the roofs of buildings, collects on paved areas like driveways, roads, runways and sidewalks is typically diverted through a system of pipes that is separate from the sewerage system. Unlike sewage, stormwater historically has not been treated, but flowed directly from streets and gutters into natural bodies of water, e.g., rivers, lakes and the ocean.
Stormwater can therefore be a form of diffuse or non-point source pollution. It can entrain pollutants, such as garbage, sediment, organic matter, heavy metals, and organic toxins, and transport them into receiving natural bodies of water. As a consequence, natural bodies of water that receive stormwater may also receive pollutants capable of irreparable environmental harm.
The amount of stormwater pollution entering into such receiving bodies of water is related to the degree of urbanization in the surrounding area and the nature of the surrounding activities. Urbanization results in the covering of land with low/no-permeability structures, such as roadways, parking lots, and rooftops, which both generate large volumes of stormwater and accumulate pollutants. Since these types of surfaces do not allow rainfall to infiltrate, they allow the accumulated pollutants to be carried into stormwater drainage systems.
In an effort to address the environmental problems posed by polluted stormwater, traps and filters for stormwater have been developed. For example, U.S. Pat. No. 5,322,629 describes a method and apparatus for treatment of stormwater by vertical filtration through a bed of high-quality leaf compost material that removes pollutants prior to discharge into a receiving water body.
A granular form of compost that improves the overall performance of such filtration systems was described in U.S. Pat. No. 5,624,576. The permeability of a filter containing such granular compost is increased, and can be maintained for an extended period of time.
The improved filter apparatus described in U.S. Pat. No. 6,027,639 features a siphon. The filter apparatus of the '639 patent utilized a sealed upper housing that included a check valve for air evacuation within the housing. An induced siphon effect pulled stormwater through the filter until air was pulled through an air inlet along the lower perimeter of the housing, disrupting the siphon. It was intended that the air stream that resulted from such a siphon-breaking event would produce turbulence and thereby clean the filter.